On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees about 1 mile above the ground as a measure of the extremity of winter.
In the middle of January, about 68 million square kilometers of the Northern Hemisphere are covered by air that cold at that level. Between about July 5 and July 20 that area shrinks to zero, and the complete absence of such air lasts only a very few days.
We are just past the point in this summer when that area is beginning to increase again. Thus, despite the fact that we have been through a couple of really hot and humid weeks this July, the return of winter has already begun in terms of this measure.
This is largely because the number of daylight hours has already begun to decrease — noticeably here in Madison. At every latitude north of 66.5N, however, the sun was out for 24 hours long on the day of the Summer Solstice, June 21. Ever since, the spectre of nighttime has been creeping poleward with the daylight decreasing ever so slightly just above the Arctic Circle. The associated nighttime cooling is greater during a longer night, resulting in the gradual increase in the amount of cold air covering the Northern Hemisphere.
So, if these past couple of weeks have been a bit too much for you, perhaps some solace can be found in the fact that the tide is already, however imperceptibly, beginning to turn.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
